Of Combustion & Work
Combustion and fire represent the earliest reactions used to perform work. It involves the burning of a fuel in the presence of an oxidizer to produce heat, potential flames and smoke, and reaction byproducts (gases). All chemical changes due to combustion are accompanied by the flow of heat energy into or out of a chemical system. Processes that can perform work take advantage of energy flow from fuels during oxidation reactions.
Industry is constantly seeking new source materials, or advanced fuels, that produce a great amount of heat at a reasonable cost. The present invention meets this need by disclosing a nanomaterial that displays unique combustion characteristics. nMx is a nanocomposite of Li3AlH6 nanoparticles, elemental Al metal nanoparticles, an amount of Ti metal, and a nanoscale organic layer. The present invention's heat of combustion, ΔH°, is higher than historical ΔH° values for burning materials typically used as advanced fuels.
Having high energy densities is an important measure for novel fuels and energetic materials. Higher energy densities indicate that a material can do more work while having a lighter mass and/or volume, which is of particularly importance as it impacts the ability to launch payloads into space, lightens the weight of aircrafts to fly faster, and gives the creation of munitions more flexibility.
Our composite falls within many classifications of advanced fuels due to its air-stability and tune-able combustion properties. We tune the chemical composition of our nanocomposite to ensure an energy density (both volumetric and gravimetric) that is suited for use as an additive to liquid propellants, a solid propellant, as additives to high energy materials, for use with explosives, pyrotechnics, for use as a source material for welding reactions, and as a source material for advanced materials production.
Metal Particles & Combustion
Due to the present invention disclosing the combustion of two distinct nanosized core metals, we address the importance of metals that burn. The combustion characteristics of metal particles make them viable alternatives to high energy fossil fuels. Iron or aluminum can be milled, or synthesized, and turned into solid fuel grains for lifting payloads into space or creating thermite hot enough to cut through steel. Some metal powders burn like hydrocarbon fuels. The energy and power of a metal-burning engine are comparable to traditional combustion engines. Metal powders often have shorter ignition delays, they burn faster, and have a higher volumetric energy density (energy per unit volume of fuel) than do fossil fuels and other organic materials. An increased volumetric energy density reduces the size of the launch vehicle and improves efficiency through better aerodynamics.
Creating metal particles with high-energy output is challenging. There are serious safety concerns in handling the starting materials and reactant products, unfeasible reaction times, high production costs, and competing reactions at the metal's surfaces. The present invention discloses nMx as a nanocomposite that overcomes such hurdles and produces novel burning profiles by harnessing the combustion properties of a homogeneous mixture of Li3AlH6 nanoparticles, elemental Al nanoparticles, an amount of Ti metal, and a nanoscale organic layer. The energetic nanoparticles are safe to handle in air for use in applications.
Aluminum Metal & Combustion
Aluminum is a valuable and versatile metal. Small aluminum particles are commonly used as an additive to propellants to increase energy output of a material or base fuel, a non-limiting example being nanoscale aluminum increasing the ignition probability of diesel fuels [1]. Al micro-particles are used in kiloton amounts for solid rocket boosters and other solid rocket propellants. While there are many applications for nanoscale aluminum materials, there are challenges with producing air stable aluminum nanoparticles having diameters smaller than 100 nm for industrial or commercial use.
When not in its neutral elemental (0) oxidation state, natural aluminum exists in a +3-oxidation state. Any process to reduce Al3+ to Al0, by gaining the three electrons, requires a large amount of energy. Because of its high reactivity, pure aluminum readily reacts with oxygen or water to form a layer of aluminum oxide or hydroxide on its outer surfaces, which explains why pure aluminum is mostly found and used in one of its many oxidized forms, non-limiting examples being Al2O3 or the mineral bauxite.
The oxide layer that forms on aluminum's surfaces greatly reduce the metal's combustion properties. The oxide blocks the core metal. This blocking slows the combustion process, and it prevents systems needing a high-energy output and a high burn rate from taking full advantage of the metal's ability to combust.
In smaller nanoparticles, aluminum's oxide layer can account for more than 70% of the nanoparticle's mass. The combustion inefficiency of aluminum metal increases for nanoparticles with diameters less than 20 nm. The oxide coating significantly lowers the nanoparticle's energy density, slows the nanoparticle combustion rate, may prevent complete aluminum nanoparticle consumption, and can reduce hydrogen absorption for storage applications.
Commercially available aluminum is very inefficient as a fuel or fuel additive. A non-limiting example being solid fuels using ammonium perchlorate, NH4ClO4, as an oxidizer for reducing aluminum metal beads bound to solid rubber. Once the rubber is ignited and starts to burn, the oxidizer reacts exothermically with the fuel, thereby forming O2− and Al3+ and producing an energy release. Oxygen diffuses into the outer layer of the metal to form aluminum oxide, Al2O3. The oxygen from the oxidizer can only diffuse by about 20 microns into the surface of a pre-coated aluminum bead. Every single aluminum bead has a coating of Al2O3 that is approximately 100 nm thick. A fair amount of the aluminum metal does not participate in combustion due to the protective oxidized layer on each bead.
This phenomenon is evidenced by the expulsion of byproducts during rocket launches. Molten aluminum chunks are ejected from the nozzle as a non-contributory element to gas expansion and thrust. When aluminum beads burn, the Al2O3 coating thickens. The additional oxide further slows combustion by reducing the amount of pure aluminum metal that participates in reduction to create an effective energy release for a combustion engine. The passivated surfaces of the present invention give a greater amount of surface reactivity for both Li3AlH6 nanoparticles and elemental Al nanoparticles for combustion events.
Recounting Li3AlH6 
The production of complex metal hydrides has a somewhat convoluted purpose, where some seek reducing agents, others seek a hydrogen storage material, and still others seek ways of exploiting the material as an advanced fuel or additive. The purpose often dictates the method of making these materials, e.g. ball milling, using varied starting materials for solvent based synthesis, and the like, thereby producing dispersions and a range of sizes within a bulk metal.
Although lithium aluminum hexahydride, Li3AlH6, sparks the imagination as an energy source, it is a difficult material to work with. There are many drawbacks to Li3AlH6 being a viable energy source. Li3AlH6 is wildly expensive, where the pricing for 1 kg of Li3AlH6 can run as high as $20,000 USD. Li3AlH6 is unstable and reacts with water and ambient gases to produce spontaneous burning. In their “natural” form most metal hydrides are not safe to handle in air. Because our nanocomposite is a first reporting of a cost-effective method for creating stabilized Li3AlH6 nanoparticles, making Li3AlH6 nanoparticles air safe and sensible for applications, we present a brief retelling of important moments for Li3AlH6.
Synthesis of Li3AlH6 was first reported by Ehrlich et al. in the late 1960's, and the thermal decomposition of LiAlH4 into Li3AlH6 was later reported by Dilts and Ashby in the early 1970's. Since then, LiAlH4 has become commonly used in industrial processes as a reducing agent to convert esters, carboxylic acids, acyl chlorides, aldehydes, and keytones into their corresponding alcohols, as drying agents, and as materials for hydrogen gas storage [2-4].
LiAlH4 and Li3AlH6, as with most of the metal hydrides, are highly reactive with water and ambient gases. These chemicals are pyrophoric and must be carefully handled and stored. When LiAlH4 is heated, Dilts and Ashby found that the thermal decomposition of LiAlH4 occurs in three steps as follows:3LiAlH4→Li3AlH6+2Al+3H2(150° C.-170° C.),  (R1)2Li3AlH6→6LiH+2Al+3H2(185° C.-200° C.),  (R2)LiH+Al→LiAl+½H2(above 400° C.).  (R3)
LiAlH4 is metastable at room temperature and partially decomposes into Li3AlH6 over very long periods of time. However, reaction steps R2 and R3 will occur at temperatures above 150° C. Meaning, to make nMx, it is vital to hold the reaction vessel below 150° C. to keep the decomposition of LiAlH4 to R1, where we further alter the surfaces of Li3AlH6 nanoparticles and elemental Al nanoparticles as needed [4].
In 1963, the heat of formation for Li3AlH6 was measured as ΔH298°=−79.40 kcal/mol. The reaction was driven in 4 N HCl in a closed bomb, where the final ΔH298° value was estimated from ΔH298° values obtained for aluminum, lithium, and Li3AlH6 [5]. Zaluska et al. reports a DSC scan for ball milled bulk Li3AlH6 having a burn event at about 240° C.-260° C. [20].
Chen et al. uses a vibrating mill technique to make nanocrystallites of LiAlH4 and Li3AlH6, where two different experiments are performed to measure H2 gas desorption and resorption [6]. Firstly, Chen et al. vibrate mills LiAlH4 along with titanium chloride anhydrous aluminum reduced (TiCl3.⅓AlCl3) for up to one hour. They report that alkoxide catalysts, such as titanium-n-butoxide (Ti(OBu)4), are highly problematic for their reversible hydrogen experiments. Chen et al. mills LiAlH4 into a micro scaled (μm) powder that contains a range of particle sizes, including dispersed LiAlH4 nanocrystals below 20 nm. However, Chen et al. could not convert LiAlH4 into both Li3AlH6 nanoparticles and elemental Al nanoparticles [6].
Secondly, in a separate experiment, Chen et al. vibrate mills 2LiH+LiAlH4 along with titanium chloride anhydrous aluminum reduced (TiCl3.⅓AlCl3) for up to one hour to create Li3AlH6. The result is TiCl3 doped Li3AlH6 nanocrystals that are granular shaped at about 20 nm. Chen et al. believe the Li3AlH6 nanocrystals are due to many factors including the presence of a separate Ti phase and a nanocrystalline Al and TixAly phase associated with the milled powder [6]. However, Chen et al. does not give PXRD data substantiating the presence of Al nanocrystals in their powder, which should give strong PXRD peaks in terms of 2Θ at ˜37°, ˜45°, ˜65°, and ˜78° as compared to other methods that report PXRD peaks for elemental Al nanoparticles [7].
In both instances, Chen et al. does not create a true nanoparticle system by stopping the decomposition of LiAlH4 at the first reaction step. Chen et al. examines the catalytic effect of a Ti0/Ti2+/Ti3+ defect on H2 formation and resorption of Li3AlH6. Chen et al. continues the reaction through Li3AlH6 decomposition, 2Li3AlH6→6LiH+2Al+3H2. They create large bundles of multiple nanocrystals that have been fused together by cold welding, where the creation and extraction of single crystal nanoparticles is impossible. Also note that Chen et al.'s nanocrystalline domains are randomly dispersed within a system that is primarily bulk material that range from about 1 μm to about 10 μm in size [6].
U.S. Pat. Pub. No. 2003/0026757 as filed by Percharsky et al. discloses the release of hydrogen gas from mechanical processing of a metal hydride at room temperature. In one instance, the starting material may be LiAlH4. The reaction takes place in the absence of any solvents to forcefully collect hydrogen gas from the storage material. The process does not use any nanostructures [8].
Choi et al. disclose the use of a Li3AlH6/LiBH4 mixture as a reversible storage medium for making hydrogen gas via ball milling. Choi et al.'s ball milling conditions are adjusted to account for unexpected reactions or changes in the original phases [9]. The process of ball milling results in aggregated nanocrystallites that morph into larger mesoscale structures that are not nanoparticles. All materials were unstable in air and were handled in a glove box under an inert atmosphere.
Varin et al. disclose the effects of ball milling on nm sized (300 nm to 90 nm±30 nm) LiAlH4. Through DSC data, they report a thermal decomposition of LiAlH4 to micron sized Li3AlH6 between 190° C.-300° C. [24]. Varin et al. does not create a true nanoparticle system by stopping the decomposition of LiAlH4 at the first reaction step. Varin et al. continues the reaction past Li3AlH6 decomposition to form LiH and Al molecules, 2Li3AlH6→6LiH+2Al+3H2 [24].
U.S. Pat. Pub. No. 2011/0165061 as filed by Yang et al. discloses a method of increasing thermal conductivity in hydrogen storage systems [10]. Yang creates a reversible reaction for making hydrogen gas by forcing Li3AlH6 and Mg(NH2)2 to liberate hydrogen under certain thermal conditions. Because metal hydrides are inherently poor thermal conductors, Yang et al. cool their ball milled particles with an aluminum film that acts as a heat sink. All of Yang et al.'s materials are air unstable.
The present invention is the first to harness the energetic properties of nanoscaled products created from the thermal decomposition of LiAlH4 for combustion processes. None of the references disclose a bottom-up synthesis that creates a homogenous material composed of nanoparticles of both Li3AlH6 and elemental Al metal that are carefully sized and passivated by a nanoscale organic layer at the first reaction step of LiAlH4 decomposition. nMx is air stabilized, contains a certain amount of Ti metal, is safe to handle, and protects and preserves the combustion properties of the same for energy applications. Therefore, there is a need for the present invention.